High yields bisulphite pulping process



p 1959 R. M. DORLANIZ; Em 2,906,659

HIGH YIELDS BISULPHITE PULPING PROCESS Filed Oct. 19, 1953 0 svnuce K HALF 8 HLLF I unto mm YIELD- 50 l l I 7 6 5 4 3 Z 'l O Y Tm: AT I551 Houqs IMYENTOH5:

R006 Efi m. bonnmu AND JAMES w. P'h/(INNE) BJWWJW A-rrokrvb-ys 2 Sheets-Sheet 2 United 2,906,659 HIGH YIELDS BISULPHITE PULPING PROCESS Application October 19, 1953, Serial No. 386,962 Claims priority, application Canada July 13; 1953 6 Claims. (Cl. 162-831) This invention relates to a process for pulping wood chips and more particularly to a semi-chemical process in which the pulping liquor is abisulphite solution.

Semi-chemical pulping processes involve an initial chemical treatment of the wood chips to soften and partially-remove the bonding material of the middle lamella followed by a mechanical refining step to separate the fibers. Semi-chemical processes are commercially attractive because it is possible to obtain an increased yield accompanied bylow chemical constunption. The lignin is extracted more selectively than with conventional-pulping processes and a far higher proportion of the hemicelluloses is retained. Alkaline cooking liquors are generally unsuitable for semi-chemical pulping as a dark coloured pulp is produced which cannot be" bleached. Neutral sulphite semi chemical processes which are carried out at a high temperature and under neutral or slightly alkaline conditions give good yields of bright, bleachable pulp where hardwoods are treated but in th'e case of softwoods or mixed hardwoods and softwoods the process is unsatisfactory as a pulp is produced having poor strength, bad colour, and high resistance to bleaching". In addition the chemical consumption is high. Acid sulphite processes are unsatisfactory as giving low strength pulps of poor brightness and bleachability. Acid sulphite processes using decreased quantities of chemicals and in particular less tree sulphur dioxide than in conventional processes have been investigated. An example era proc ess of this nature is Belgian Patent 496,841 which issued July 31, 1950, on an application filed by Azinda Brevetti Industriali, The pulp produced by this process does not provide adequate strength and brightness in a reasonable pulping period. 7

The object of this invention is in general to provide an improved semi-chemical pulping process, capable of providing, even in the case of softwoods or mixed softwoo'ds and hardwoods a high yield of pulp possessing good strength and brightness, and requiring low refining power.

It has been found in accordance with this invention that a pulp having desirable properties can be produced by cooking wood chips with a solution of a bisulphite of a substance selected from the group consisting of sodium, potassium, and ammonium. The cooking liquor used in accordance with this invention contains substantially no rates Patent "Ice follows. In the case of conventional sulphite processes.

the sulphur dioxide content is reportedlin terms of free S0 and combined 80 1 In fact theamount of sul true fr ee sulphur dioxide. The chemical step is' followed by a mechanical refining treatment to separate the fibers. The nature of the cooking liquor used in accordance with this invention, and the essential distinction between phur dioxide reported as combined is'the' amount theoreti-' cally combining with the base as monosulphite. The amount ofvsulphur dioxide reported as free S0 will include a'percentage" of sulphurdioxide which is actually in combinationtofo rm the bisiilphite'and some truefre'e' sulphur dioxide; The amount of theffree sulphur dioxide which is in combination to forrii the bisulp'hite will be equal to the'sulphur' dioxide reported'as combined and the true freesulphur dioxide can-be calculated by-subtracting the valu'e'for com'bineil sulphur dioxide'from-the value for free sulphur dioxide.- For example a solution reported as having 1% combined-SO 'and'5'% free will actually include2% of bisulphiteand 4%oftrue' free 805;; It is contemplated'in"accordance-with this invention that the solution contain equal'proportions of combined S0 and free'SOg, as ordinarilyreported: This meansthat all of the S05 will actually'be incombiriationwith'the base as bisulphite and there will"be substantially no true free sulphur dioxide: It'i's emphasized that the bisulphite process'of this invention must bedistinguished-from the processes commonly referred toas bisulphite processes and'wliich in'volve the use of-sulphur dioxidein excess of the amount theoretically necessary' to combine with the base of bisulphite. Bisulphite is sometimes used as a synonym for the sulphite process.

The process of this invention is characterized by an initial pH of betweeu 4"and 5. During pulping organic acids are released from the wood and-these cause the pH to-fall to about 315; but in no casedoes the pH decrease below 3. This may be contrasted with sulphite acid solutions which normally have a pH of about 1:5 and which are strongly buttered, the pH changing littleover a considerable range of compositionsand' concentrations.

It is'n'ecessary that the-base used in the process of this invention becapable offorming a soluble monosulphite. Thus calcium is unsuitable as in the absence of excess sulphur dioxide there would beprecipitatiorr of the base. Sodium; potassium and ammonium form soluble 'sulphites and it is therefore possible to use solutions free of excess sulphur dioxide.

one of the advantages of the process of this invention in comparison with'processes such as the acid sulphite process is that it' is possible to' raise the temperature to as high" as 155'. C. without producing a burned pulp. As a consequence the reaction proceeds at several times the rate of a similar process carried out at aj maximum of, say 140 C. In the acid sulphite process it is necessary to operate within a maximumof about, C. to avoid excessivedegradationofthe; cellulose. This possibility of using an increasedtemperature provides compensation for the reduced rate of Ieactiondue; to the absence of true free sulphur dioxide; i n

It might be expected that at'ra"temperature 01315 5} C. there would be an excessive attack 3 onthe -hemicelluloses. In fact it has been'foundrthattherewis unexpected decrease in the extent to I whichihemicellulos'esare attacked in comparison with -alsulphite process carriedout, ata Patented Sept. 29, 1959 lower temperature and having a comparable yield. It i present as a definite chemical compound, will remain in the liquor phase from whence it can be recovered by conventional methods. In addition the retention of the cooking chemical in the waste liquor resulting from the process of this invention makes it feasible to reuse the waste liquor. Another advantage partially dependent on the factors which have been discussed and probably due partially to other causes which have not yet been fully investigated is the empirical fact that the general properties of the pulp produced in accordance with this invention are highly desirable. The pulp has good strength and brightness at high yields and requires lower refining power than pulps produced by the sulphite process at comparable yields. A soft chip is produced at a high yield. Microscopic examination of the pulp together with fiber classification has shown that the pulp after refining had been fully defibered with little cutting or breaking of the tracheids. The fibers were cylindrical rather than the usual flat form of pulp fibers. The refined pulp exhibited a remarkably fast drainage rate.

The properties of the pulp will depend to a certain extent on the yield. It is preferred that cooking be continued to a yield of 65 to 75% which is about 20% higher than a comparable complete chemical cook. This range may be extended within the ambit of this invention to give special qualities to the pulp. In Example 1 an example is given of a cook in accordance with this invention.

EXAMPLE 1 The digester was charged with 1 part by weight of spruce chips and 5 parts by weight of a liquor formed by dissolving sufficient sodium metabisulphite in water to provide a total S content of 2.5%. The chips were digested under pressure at a temperature which was raised to 110 C. during a 2 hour period. The temperature was raised further to 155 C. during a 2 /2 hour period and the temperature was maintained at this figure for a further period of 1% hours. The initial pH was 4.7 and the final pH 3.5. The yield was 74%. The cook was blown and the softened chips were separated from the liquor and washed and then passed through a disc refiner which separated the fibers to form the pulp. Particulars of the properties of the pulp will be apparent from Table 1 below under the heading Cook No. 6. While the above example shows a liquor/wood ratio of 5:1, experimental results indicate that there should be no difliculty in utilizing a ratio of 3:1 corresponding to 12% NaHSO B/wood. The cooking liquor can also be prepared from a solution of soda ash and sulphur dioxide gas.

The equipment and procedures used in Example 1 and in obtaining the other test results reproduced in this specification will now be briefly described. The laboratory digester is a stainless steel pressure vessel of about 2 cubic foot capacity with a bolted top and a cone bottom. It is arranged for circulation, the liquor being introduced near the top, striking a distributor and then falling by gravity over the chips, after which it is removed through a screen in the cone -and a pipe near the bottom of the cone. Temperatures are measured in Pulp the inlet and outlet pipes. The liquor is circulated by a pump through a heat exchanger, then into the top of the digester again. There is a complete turn-over of the liquor about every half-minute.

Cooking liquor is made up in glass carboys holding about 110 lbs. of liquor. Sulphite liquor is made from calcium or sodium carbonate, and S0 gas from a cylinder. The constituents are added by weight, then an accurate analysis made by the standard Palmrose method. For cooks in accordance with this invention the same procedure may be followed, or commercial sodium metabisulphite may be dissolved in water. The same is true for neutral sulphite cooks, but again the proportions are different. The liquor to wood ratio may be varied, but for sulphite cooks follows the usual commercial practice at 5.0. For cooks in accordance with this invention 4.0 is usually used, while for neutral sulphite 2.7 is common.

For spruce,.30 lbs. of chips are used for a charge, adjusted to'40% moisture, equivalent to exactly 18 lbs. of dry wood substance. (Moisture is determined by drying a sample at C.) These chips are charged into the digester with some hand-packing, and the lid bolted on. The cold liquor is added and the pump started, then steam is admitted to the heat exchanger fast enough to bring the temperature at the inlet up at a steady rate, according to a pre-determined schedule. At moderate rates of heating, the temperature drop to the outlet is about 3. After the maximum temperature is reached, it is maintained for the time specified; the maximum pressure is also specified, and maintained by an automatic relief valve. Then the steam is turned off, the pressure partly relieved and the 3-inch blow-valve at the bottom of the digester is opened. The softened chips are partly blown out into the blow tankabout four times the capacity of the digester-and if they are well-cooked they are largely reduced to pulp. The remainder are washed out and the whole washed with water in the blow tank.

The drained mass is then transferred to a dilution tank in the case of pulp, where the yield is determined, and the pulp is then screened to remove uncooked pieces of wood, etc. Hard chips are taken to the refiner.

Refining is conducted as follows: The high-yield or hard chips are defibred in a standard 24" Bauer'doubledisc refiner with plates of standard design. This is a commercial model, equipped for handling small charges of chips in the laboratory. The amount available from a cook varies, but at 70% yield using spruce, the charge to the refiner will be about 12.5 lbs. The chips are spread uniformly over a feed belt which drops them into the refiner at a constant rate. Standard conditions of plate setting, i.e. distance between the revolving plates, and refining consistency (water is added to the pulp between the plates), are used. Between refining passes the pulp consistency is increased to approximately 20% RD. by pressing in cloth bags in a cold press. The

cloth used is fine enough to retain the fines present in the refined stock. The number of refining passes used is determined by the appearance of the pulp examined on a blue-glass-it is refined until it appears shive-free. Complete defibration may take from 2 to 4 passes, but usually not more than three, and the total power consumed, measured by an integrating watt-meter, is commonly between 10 and 20 horse-power-days per ton of In Table 1 a comparison is made between lime base sulphite cooks, soda base sulphite cooks and a bisulphite cook in accordance with this invention. In each case the wood chips being treated were spruce and the ratio of 5:1 of liquor to wood was used.

Table 1 Cook No 1 2 3 4 5 6 Type of liquor Lime-base... Sulphito-- Soda-base... Su1phite Bisu1phite Bisulphite. Free 80;, per 3.01-- 3 02 3.04 T 3.04 1.27 1.25. Combined 80;, percent... 0.80 0.82 .25. Temp, 0. 135 135 155. Time to 110, hrs 2 2 2. Time to maximum, hrs 2 2 2.5. Time at maximum, hrs-.- 1.5 1.5. Total time 5.5-- 5 6. Initial pH Approx"..- 1.5 4.7. Final p Approx".-- 1.5 3.5. Refiner passes 2 3 2. H.P. days/ton 9.5. 14.7 11,4. Freeness. C.S.F 705 710 710. Yield, percent 62 69 74. Brightness..- 42 I 44 56. Initial burst factor 44 so 52. Initial tear factor 7 7 104. Pentosam. 4.2.. 4.3. 5.5.

1 Using the process lmown as vapurge, described in United States Patent 2,640,774.

In Table 2 a comparison is made between a cook in accordance with this invention (No. 8) and a cook at'the same temperature and using a slight excess of true free sulphur dioxide (No. 7). Both cooks were conducted using vapurge to assist penetration. This table emphasizes the distinction between the process of this invention and conventional sulphite processes. In particular the unsatisfactory physical properties of the product obtained using an excess of sulphur dioxide will be noted.

Table 2 Cook No 7 8 Free S02- 1.00 1. 27 Combined S02 0.50 1. 26 Maximum temperature, 155 155 Time to 110, hours 1 1 Time 110, maximum hours. 1 0. Time at maximum, hours..- 0.5 2. 5 Total time, hours 2. 5 4.0 Initial 11H 2. 5 4. 7 End oil 2. 6 3.7 Yield, percent 74. 5 68. 1 Brightness. 39 50 Lignin, percen 29.2 17.1 Freeness, G S 685 670 Initial burst fact 11 66 Initial tear factor; 110 85 Initial fold factor 4 460 In Table 3 the result of increasing the temperature with a cook in accordance with this invention is shown. It will be noted that at 155 C. a combination of desirable properties is obtained including low refining power, low cooking time and good physical properties. It is apparent that these desirable properties are retained even at a cooking temperature as high as 165 C.

Table 3 EFFECT OF TEMPERATURE Cook No 9 10 11 12 13 14 15 Free S0 1. 26 1. 26 1. 22 l. 22 1. 22 1. 27 1. 27 Comb. 80 1.30 1.30 1.26 1. 26 l. 27 1. 26 1.27 Max. temp., C- 115 125 135 145 155 155 165 To 110, hours 2 2 2 2 2 1 1 110, max, hours 0.5 l 1. 5 1. 5 2 0. 5 1. 5 At max, hours- 7. 5 7 6. 5 6.5 3 2. 5 1 Total time, hour 10 10 10 10 7 4 3. 5 4. 5 4. 4 4. 2 4. 2 4. 4 4. 7 4. 8 3.5 3.3 3.2 3.1 3.4 3.7 3.7 95. 0 87. 0 80. 9 75. 2 63.0 68. l 65. 1 Brightness 58. 3 56. 5 53. 3 47. 5 49. 2 49. 8 50. 2 Lignin, percent 24. 9 23. 5 20. 8 15. 7 13.1 17. l 15. 6 Freeness, O.S.F 725 735 585 665 670 670 680 Init. burst fact0r 4 5 61 64 69 66 70 Init. tear factor. 145 80 115 70 75 85 80 Fold 0 0 75 480 9 460 640 Refiner passes 4 4 3 2 2 2 2 Refiner, H.P.D./ton 19 8 20.2 14. 5 8. 0 9. 4 10. 6 11. 1

The difference between cooks in accordance with this invention conducted at an initial pH of 4.2 to 5.0 and a final pH of 3.3 to 4.5 and cooks conducted in accordance with conventional procedures at an initial pH of about 1.5 to 2.5 is further illustrated in Figure 1 which shows yield plotted against initial burst factor. The cooks in accordance with this invention are designated as high pH and conventional cooks as low pH. The results providing the basis for Figure 1 were obtained with spruce pulps. It will be noted from Figure 1 that throughout the range of high yield pulps the process of this invention gives an initial burst factor higher by about 15 points.

It is appreciated that full length cooks have previously been conducted on an experimental basis using a solution of sodium bisulphite. In each case however the properties of the pulp were found to be unsatisfactory and no further investigation appears to have been made. The present invention is to be distinguished from such processes as the advantages obtained in accordance with this invention are fully achieved only in the case of a semichemical cook in which an initial chemical treatment is followed by a mechanical refining treatment. Thus it will be noted from Figure 1' that with a'decreased yield the advantage in initial burst factor presented by the process of this invention decreases until a point is reached at about 52% yield whereit presents no advantage over a conventional cook. A full length cook at a high pH is also unattractive as a cooking time of about 9 hours is required for 52% yield as compared with a total time of about 4 hours for an %yield.

Figure 2 is a similar plot of tear factor against yield. It is well known in the pulp industry that'burst and tear are to some extent inverse; that is, when pulp is subjected to a treatment such as heating, the burst commonly rises while the tear declines. It will be seen from Figure 2 that this inverse relationship holds with both types of pulp over most of the range, but the pulps produced in accordance with this invention are generally higher in tear, rather than lower as might have been expected from their higher burst. In other words an enhancement has been obtained in the opposed properties of burst and tear. These results point to the unique nature of the pulp produced in accordance with this invention. The fibres appear to be softer than those obtained at a lower pH but no satisfactory measurement of this quality has yet been developed which would properly express this property.

Figure 3 shows a plot of pulp yield against cooking time for cooks conducted in accordance with this invention.

We claim:

1. A semi-chemical process for forming pulp consisting essentially of the steps of cooking submerged wood chips for about 2 /2 to 7 hours to weaken the inter-fiber bonding with a solution of a bisulphite of a substance selected from the group consisting of sodium, potassium and ammonium and having an initial pH of not less than 5, the pH being not less than 3 during said cooking, said solution having substantially no sulphur dioxide in ex cess of the stoichiometric amount necessary to form said chanically refining the softened wood chips to separate I the fibers.

cally refining the softened wood chips to separate the 2. A semi-chemical process for forming ulp consisting essentially of the steps of cooking submerged wood chips for about 2 /2 to 7 hours to weaken the interfiber bonding at a temperature rising to about 155 C. with a solution of a bisulphite of a substance selected from the group consisting of sodium, potassium and ammonium, said solution having substantially no sulphur dioxide in excess of the stoichiometric amount necessary to form said bisulphite, the middle lamellae of said wood chips being softened and partially dissolved to enable the fibers readily to be mechanically separated but said fibers being coherent during the blowing of the cook, followed by mechanically refining the softened wood chips to separate the fibers.

3. A semi-chemical process for forming pulp consisting essentially of the steps of cooking submerged wood chips for about 2 /2 'to 7 hours with a sodium bisulphite solution having a total sulphur dioxide content of about 2 /z% and a combined sulphur dioxide content of half the percentage of total sulfur dioxide and having an initial pH within the range 4 to 5, the pH being not less than 3 during said cooking, said cooking step being continued until the middle lamellae of said wood chips are softened and partially dissolved to enable the fibers readily to be mechanically separated but said fibers being coherent during the blowing of the cook, followed by mechanically refining the softened wood chips to separate the fibers;

4. A semi-chemical process for forming pulp consisting essentially of the steps of cooking submerged wood chips for about 2 /2 to 7 hours 'with a sodium bisulphite solution having a total sulphur dioxide content of about 2 /z% and a combined sulphur dioxide content of half the percentage of total sulphur dioxide and having an initial pH within the range 4 to 5, the pH being not less than 3 during said cooking, the cooking being conducted at a temperature rising to about 155 C. until the middle lamellae of said wood chips are sufiiciently softened and partially dissolved to enable the fibers readily to be mechanically separated but said fibers being coherent during the blowing of the cook, followed by mechani- 5. A semi-chemical process for forming pulp consisting essentially of the steps of cooking submerged softwood chips for about 2 /2 to 7 hours with a sodium bisulphitesolution having a combined sulphur dioxide content of half the total sulphur dioxide content and having an initial pH within the range 4 to 5, the pH being not less than 3 during said cooking, the cooking being conducted at a temperature rising to about 155 C. until j the middle lamellae of said wood chips are sufliciently softened and partially dissolved to enable the fibers readily to be mechanically separated but said fibers being coherent during the blowing of the cook, followed by mechanically refining the softened wood chips to separate the fibers.

6. A semi-chemical process for forming pulp consisting essentially of the steps of cooking submerged hardwood chips for about 2 /2 to 7 hours with a sodium sulphite solution having a combined sulphur dioxide content of half the total sulphur dioxide content and having an initial pH within the range 4 to 5, the pH being not less than 3 during said cooking, the cooking being conducted at a temperature rising to about C. until the middle lamellae of said wood chips are sufiiciently softened and partially dissolved to enable the fibers readily to be mechanically separated but said fibers being coherent during the blowing of the cook, followed by mechanically refining the softened wood chips to separate the fibers.

References Cited in the file of this patent UNITED STATES PATENTS 1,880,042 Richter Sept. 27, 1932 2,425,024 Beveridge et al. Aug. 5, 1947 2,454,533 Walter Nov. 23, 1948 2,749,241 Marpillero June 5, 1956 FOREIGN PATENTS 12,149 Australia Apr. 19, 1934 of 1933 496,841 Belgium Nov. 3, 1950 504,423 Belgium July 31, 1951 OTHER REFERENCES Yorston: Dominion Forest Service Bulletin 97, Ottawa (1942), pp. 34, 35, 40 and 45.

UNITED STATES PATENT OFFICE CERTIFICATE" OF CORRECTION September 1959 Patent No. 2 906, 659

Rodger M. Dorland et a].

Iiiis hereby certified that error appears, in the printed specification vq'equifIIIgT WTTITE SEIGTTGTBEYS" Q9 ik boy mus ered Tare; OI'I'EUD Patentsfio'ufread as corrected below.

Column 5 and 6; Table l, the heading thereof should appear as shown below instead of as in the patent:

Type of liquor Lime-base sulfite Soda-base sulfite Bisulf'ite Cook No l 2 3 4 5 etca column 5, Table 2, first column thereof, under the heading Cook N06 6 a o a, line 5 thereof, for "Time ll maximum hours" read Time, 110 to maximum, hours column 6, line '72, for "less" read more colwrm 8 line 19, for "sulphite" read bisulphite Signed and sealed this 10th day of May 1960 (SEAL) Attest:

ROBERT C0 WATSON KARL 1-1,. AXLINE Commissioner of Patents Attesting Officer 

1. A SEMI-CHEMICAL PROCESS FOR FORMING PULP CONSISTING ESSENTIALLY OF THE STEPS OF COOKING SUBMERGED WOOD CHIPS FOR ABOUT 21/2 TO 7 HOURS TO WEAKEN THE INTER-FIBER BONDING WITH A SOLUTION OF A BISULPHITE OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF SODIUM, POTASSIUM AND AMMONIUM AND HAVING AN INITIAL PH OF NOT LESS THAN 5, THE PH BEING NOT LESS THAN 3 DURING SAID COOKING, SAID SOLUTION HAVING SUBSTANTIALLY NO SULPHUR DIOXIDE IN EXCESS OF THE STOICHIOMETRIC AMOUNT NECESSARY TO FORM SAID 